Inducers are typically utilized as the first pumping element of centrifugal and axial flow pumps to lower the inlet pressure at which cavitation results in pump head (discharge pressure) loss. Inducers include blades that are designed to operate in a passage with a small positive incidence angle between the fluid angle relative to a blade pressure side angle as the fluid enters an area between the operating blades know as a blade row. A tip clearance between the blade tip and a wall of the passage is necessary to allow the blade tip to operate within the passage.
Camber is then added to blade geometry after the fluid is captured in the blade row to add work to the fluid raising its tangential velocity and static pressure. The small positive incidence angle near the blade tip is selected based on the gross uniform axial velocity. Because of boundary layer losses and a back flow at the blade tip that flows through the tip clearance, the actual incidence angle relative to fluid in the tip clearance is larger due to the momentum exchange and mixing resulting in lower axial velocity of the fluid. The larger incidence angle results in a larger differential pressure across the blade tip near the leading edge, which, in turn, results in larger back flow through the tip clearance. This dynamic feedback mechanism develops a queasy steady state condition at most inlet pressure and flow rate vs. operating speed conditions.
At operating speed, the velocity of the back flow through tip clearance is sufficient to lower the local static pressure below the fluid vapor pressure resulting in vapor bubbles forming in the high velocity region, which collapse as the velocity is dissipated, and the local static pressure increases. Dynamic instability including higher order rotating cavitation (HORC) and higher order surge cavitation (HOSC) appear as pressure oscillations at inlet pressures. HOSC and HORC, occur when the tip vortex (tip clearance) cavitation cavity is approximately equal to 65% of the blade spacing at the blade tip. The frequency of the higher order oscillations are typically on the order of 5 to 8 times shaft speed, depending on the number of inducer blades and other features that make-up the inducer geometry. The dynamic instability only occurs within a limited flow rate verses speed range, which suggests that it is incidence angle sensitive. At low flow rates, the incidence angle is large resulting in a large cavitation cavity at all inlet conditions. At high flow rates, the incidence angle is small resulting in a small cavitation cavity at all inlet conditions.
Head break down results from blockage due to the cavitation sheet that originates on the suction side of the blade leading edge when the local static pressure falls below the propellant vapor pressure. Leading edge cavitation sheets typically progress from alternate blade cavitation to rotating blade cavitation to gross head loss as the inlet pressure is decreased. Problems associated with these characteristics are avoided by maintaining the margin on break down conditions. The HOSC and HORC are not a result of the cavitation sheet that springs from the blade leading edge, but are instead a function of the tip clearance back flow and the tip vortex cavitation cavity length.
As a result, there is an unmet need in the art to minimize the tip vortex cavitation cavity by suppressing the back flow through the tip clearance.